1. Field of the Invention
The present invention relates to a wavelength converter for converting the wavelength of input signal beams into different wavelength, and more particularly, to a direct optical modulation (DOM) type wavelength converter for directly converting the wavelength of optical signals without a process of converting the optical signals into electrical signals.
2. Description of the Related Art
Wavelength converters are essential functional devices in constituting a wavelength division multiplexing (WDM) optical network to efficiently utilize the network. Wavelength converters are obtained by using various methods; among these, a method of using cross gain modulation (XGM) and cross phase modulation (XPM) of a semiconductor optical amplifier (SOA) is considered to have the highest possibility of realization, because the method has advantages such as simplicity and high conversion efficiency.
FIG. 1 illustrates a wavelength converter by an XGM method. In this case, an input data signal having a wavelength λ1 and a continuous wave (CW) laser input signal beam having a wavelength λ2, which is the wavelength to be converted, are incident into an SOA 20. The intensity of the CW laser input signal is modulated by the input data signal and a converted signal having a wavelength λ2 is output. Here, the converted signal is output as an inverted data signal of the input data signal. In addition, an XPM type wavelength converter utilizes an SOA circuit in which SOAs are integrated in both sides of a Mach-Zender interferometer.
A wavelength converter using semiconductor lasers converts the wavelength of an input signal beam into the wavelength of a laser emission signal beam by using a DOM technique which generates or eliminates a laser emission according to the intensity of the input signal beam. Such a wavelength converter performs as the wavelength converter described with reference to FIG. 1 while having a simple structure as shown in FIG. 2. Here, the wavelength converter of FIG. 2 outputs a converted signal having a wavelength λ2 according to only an input data signal having a wavelength λ1 to a laser diode (LD) 40.
Although such a wavelength converter using semiconductor lasers has the advantages of a simple structure and a high conversion efficiency, the wavelength converter requires an input signal beam having high intensity. More specifically, most of the laser wavelength converters require an input signal beam having an intensity of about 0 to 10 dBm. Even though structures for integrating an SOA at an input of an LD in order to lower the required intensity of an input signal beam have been presented; however, laser wavelength converters still require an input signal beam of over −4 dBm.